absorption

Absorption is how a drug gets from where you put it (a swallowed pill, an injection under the skin, a patch on your arm) into the bloodstream, which is the body's delivery highway. Until a drug reaches the blood, it cannot circulate to the tissues where it needs to act. Think of it as the drug clearing customs at the border before it can travel through the country.

Mechanistically, absorption usually means crossing biological membranes — the layers of fatty cells that line the gut, lungs, and skin. Most drugs cross by passive diffusion, slipping through the membrane from a region of high concentration to low concentration. The speed and completeness of absorption depend on the drug's chemistry (fat solubility, degree of ionization, molecular size), its formulation (tablet versus solution), and the absorbing surface (the gut's vast lining absorbs more readily than the small area under the tongue).

Absorption is the first letter of ADME (absorption, distribution, metabolism, excretion). It directly shapes bioavailability — the fraction of an administered dose that actually reaches the systemic circulation intact. A drug given intravenously skips absorption entirely (it is placed directly in the blood), which is why intravenous dosing is the benchmark of complete, immediate availability.

A common misconception is that swallowing a drug guarantees it enters the blood. In reality, absorption from the gut can be slow, incomplete, or destroyed by stomach acid and gut enzymes before it ever crosses the wall — which is one reason some drugs must be injected instead.